Making the background see-through, with a recticle staying visible
You can use this kind of node setting:



The principle is the following:


Your image is used as diffuse
But it is also used as input to drive the mix shader
This is driven by the white parts here: the more white the more the mix shader will use its second input which is here a glass


Using a glass shader can help to have this kind of zoom like effect, just changing the IOR value of the node: